The southbound lane of State Highway 1 at Meremere has been cleared while the northbound lane remains blocked.
This morning's wet weather has seen numerous crashes around Auckland and Hamilton, leaving at least one person critical and several others with serious and moderate injuries.
Waikato police Senior Sergeant Phil Ruddell said motorists could continue to expect delays this morning as emergency services work to clear the six vehicles involved in the crash at Meremere.
He said the 6.40am crash occurred after a car pulling out of Island Block Rd to turn north was hit by a vehicle travelling south. This shortly afterwards caused another crash by people travelling in the northbound lane.
"Following on from that other vehicles travelling south have been unable to stop and avoid vehicles that have stopped in front of them.
"This has caused two nose-to-tail type crashes involving two vehicles and two trucks.
"So truck hit a vehicle, the second truck hit the first truck and a trailer on the second truck has hit another vehicle."
All up six vehicles - three cars, one ute and two trucks - were involved.
"Police and emergency services from both the Waikato and Counties Manukau districts have attended. They have managed to keep at least one north and southbound lane open, however there is considerable traffic congestion.
"It will take a while for the backlog to ease."
The driver of the car that was hit was taken to Middlemore Hospital with moderate injuries.
By 9.45am, the northbound lane was fully open but one lane was still blocked in the southbound lane.
He said the crash didn't appear to involve onlookers, or rubberneckers, as originally thought, it was simply cars not being able to stop in time for affected vehicles in front of them.
"It is a timely reminder for motorists to adjust their following distances when weather conditions are poor ... you should be driving so as to be able to stop short of any incident that is happening in front of you, even if you are on the expressway going 100km/h.
"Increase your following distances in wet weather."
Meanwhile, one person was critically injured and another seriously injured after a serious crash in Whitford, Auckland.
Auckland Westpac Rescue Helicopter responded to the crash between two vehicles at 8am.
"The crew responded using the Rapid Response Vehicle.
"Attending Helicopter Emergency Medical Service Doctor Gareth Richards, accompanied a female patient in a critical condition, by ambulance to Middlemore Hospital.
Ruddell said the crash started after two vehicles collided near the intersection with Bryce St, with one catching fire, and then causing other parked vehicles to catch fire.
"The fire has also caused damage to surrounding vehicles. The cause of the crash is still being investigated."
The Hamilton Fire Brigade urged motorists to pay attention when driving after the crash damaged six cars.
"A reminder to everyone, that when you are driving, please keep your eyes on the road. This was caused by a nose to tail this morning resulting in 6 cars being severely damaged. Crews arrived to find 2 cars fully ablaze with a 3rd also damaged by fire," they wrote on their Facebook page.
Earlier today a crash on Auckland's Harbour Bridge blocked a citybound lane at 6.25am.
The crash happened on the southbound side and traffic between Greville Rd and Constellation Dr is moderate to heavy.
The cars have now been cleared from the lanes and are in the centre median strip.
In total, there were four crashes in under an hour in wet conditions, Ruddell said.
After the fiery prang on Tristram St, the driver of a vehicle fell asleep at the wheel on Old Farm Rd and crashed into some parked cars.
And at 7.45am on Dinsdale Rd, Hamilton, two cars crashed blocking the road for a short time. There were no injuries.